This invention relates to a method for manufacturing a semiconductor substrate used for the manufacture of semiconductor devices, more particularly power transistors.
Recent demands for the semiconductor products are improved performance and reduced cost.
Hitherto, the semiconductor substrate that is used for a power transistor has been a commonly termed one side lap wafer or OSL water. The OSL wafer is either of NonN.sup.+ -type or PonP.sup.+ -type depending upon the type of doped impurity. To manufacture such semiconductor substrates, two methods are alternatively employed. In one of the methods impurities are diffused into a thick raw semiconductor substrate from both major surfaces at the same time, thus forming deep diffusion layers. Then, the diffusion layer on one major surface is removed by lapping or polishing, followed by mirror surface finishing to obtain the substrate product. In the other method, a raw semiconductor substrate slightly thicker than the product substrate is subjected to a step of forming on both major surfaces high impurity concentration layers of 20 .mu.m thick and having the same conductivity type as the substrate. Then the high impurity concentration layer on one surface is removed, and an insulating film is formed to cover the entire surfaces. Subsequently, the substrate is subjected to a high temperature heat treatment to form a deep diffusion layer only in one surface, thus completing the product.
However, the substrate manufacture methods mentioned above have the following drawbacks.
(1) In the first-mentioned method, in which the diffusion is done from both major surfaces, a thick substrate has to be used as the starting material, and the cost is high.
(2) A long time is required for step of removal on one major surface and also for the mirror surface finish.
(3) The operating process is cumbersome.
(4) In the second-mentioned method, in which the diffusion is done from only one major surface, at the time of the polishing step and also for chemical ethcing for the removal of the impurity layer on one major surface, an inclined edge surface of the substrate is liable to result, thus impairing the parallelness.
(5) Since the protection by insulating film lamination is provided only as one layer or layers of the same character, at the time of the diffusion in one major surface an abnormal diffusion layer is liable to result due to intrusion of high concentration impurity into the other surface.
As a measure for solving the above problems, it may be thought to use a vapor growth wafer. Again in this case, the substrate as the starting material is expensive, and this cannot be an absolute measure.